This invention pertains to a method for producing an inner-skinned functionalized tube, whereby a functionalized coating is provided on the lumen-defining surface, i.e., the inner surface, of a hollow narrow tube having an inner diameter less than or equal to about one inch. In one embodiment, this invention pertains to a method for imparting a discriminating layer upon the lumen-defining surface of a hollow fiber, producing a hollow fiber membrane.
The following methods are known for coating the external surface of an article, i.e., for producing an exterior-skinned functionalized tube: (1) spreading the coating on the surface using paint brushes, rollers, paint pads or doctor blades: (2) spraying the coating onto the surface using, for example, air-fed spray, airless spray, hot spray or electrostatic spray: (3) flow coating the surface, for example, by dipping, curtain coating, roller coating, or reverse roller coating: and (4) electrophoretic deposition. For a discussion of these application methods, see L. Lambourne, Paint and Surface Coatings: Theory and Practice, Editor: Halsted Press: New York, 1987 (pages 39-40).
Methods for providing a functionalized coating on the internal surface of a larger diameter tubular article, by providing an object useful to distribute the coating, such as a ball, bladder, doctor blade, or rag, are also known. In particular, U.S. Pat. No. 3,676,193 teaches drawing a coating bob up the interior of a tubular cylinder to apply a polymeric coating. U.S. Pat. No. 3,657,402 teaches the use of a gravity dropping coating bob to apply a coating fluid to the bore of a tubular reverse osmosis module. U.S. Pat. No. 3,457,170 teaches filling a lumen with a casting fluid and inserting and moving a doctor blade longitudinally to form a film upon the lumen-defining surface.
The above methods cited for providing a functionalized coating on the internal surface of an article suffer the following disadvantages. First, while a bob may suitably coat the internal surface of a tube having a large inner diameter, it proves impractical for coating a tube having a smaller inner diameter, e.g., a tube useful in membrane applications. Second, when a bob drops through a vertically disposed tube, a non-uniform coating may result due to dripping. Third, a bob does not permit the uniform coating of the lumen-defining surface of a tube wherein the inner diameter varies along the length of the tube.
Methods for providing a functionalized coating on the internal surface of a hollow fiber by interfacial polymerization are known. However, interfacial polymerization methods are limited to coating materials that form condensation polymers. This limits the coating materials available.
Providing a functionalized coating on the lumen-defining surface, in contrast to coating the outer surface of a hollow fiber, permits the use of the fiber in high pressure tube-side-feed reverse osmosis applications. In particular, a high pressure tube-side-feed stream presses a lumen-side discriminating layer onto the lumen-defining surface, whereas it would push and eventually detach or breach a discriminating layer from the outer surface of a hollow fiber. Tube-side-feed arrangements afford significant advantages. First, tube-side-feed hollow fiber membranes resist fouling, often problematic in liquid processes. Second, tube-side-feed hollow fibers serve as a pressure vessel, making an external high pressure vessel unnecessary.
Those in industry would find great advantage in a method for imparting a thin, uniform functionalized coating to the lumen-defining surface of a narrow tube. In particular, the method should permit the coating of a tube wherein the cross-sectional configuration of the lumen is circular, elliptical, or irregular. The method should permit the coating of a tube wherein the inner diameter varies along the length of the tube. The method should permit the coating of a lumen-defining surface of a hollow fiber membrane, imparting it with a permselective layer, or a protective layer therefor.